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The Performance.now() method returns a DOMHighResTimeStamp, measured in milliseconds, accurate to one thousandth of a millisecond.

The value represented by DOMHighResTimeStamp varies according the context. Bear in mind the following points:

  • In any context, the value is the value from t0 of the main context (the PerformanceTiming.navigationStart property) — main context, or any type of worker spawned from the main context (or another worker.)
  • In dedicated workers created from a Window context, the value in the worker will be lower than performance.now() in the window who spawned that worker. It used to be the same as t0 of the main context, but this was changed.
  • In shared or service workers, the value in the worker might be higher than that of the main context because that window can be created after those workers.

Syntax

t = performance.now();

Example

var t0 = performance.now();
doSomething();
var t1 = performance.now();
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.")

Unlike other timing data available to JavaScript (for example Date.now), the timestamps returned by Performance.now() are not limited to one-millisecond resolution. Instead, they represent times as floating-point numbers with up to microsecond precision.

Also unlike Date.now, the values returned by Performance.now() always increase at a constant rate, independent of the system clock (which might be adjusted manually or skewed by software like NTP).

Specifications

Specification Status Comment
High Resolution Time Level 2
The definition of 'Performance.now()' in that specification.
Editor's Draft Stricter definitions of interfaces and types.
High Resolution Time
The definition of 'Performance.now()' in that specification.
Recommendation Initial definition

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 20.0 webkit
24.0 [1]
15.0 (15.0) 10.0 15.0 8.0
on Web workers 33 34.0 (34.0) ? ? ?
now() in a dedicated worker is now separate from the main context's now(). ? 45.0 (45.0) ? ? ?
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support 4.0 25.0 15.0 (15.0) 10.0 No support 9 25.0
on Web workers ? (Yes) 34.0 (34.0) ? ? ? (Yes)
now() in a dedicated worker is now separate from the main context's now(). ? ? 45.0 (45.0) ? ? ? ?

[1] Windows versions of Chrome 20 through 33 return performance.now() only to millisecond precision.

See also